Analog to digital converters (ADCs) are utilized in a variety of electronic devices and systems to transform an analog signal to a digital signal. One ADC architecture commonly used is the delta-sigma ADC. The differentiating aspects of the delta-sigma ADCs include the use of oversampling in conjunction with decimation filtering and quantization noise shaping. Advantageous characteristics of the delta-sigma ADC include high resolution and high stability. Due to these characteristics, delta-sigma ADCs are frequently chosen for use in audio systems, such as hearing devices, microphones, and the like.
Due to the low operating voltage of many devices which employ the delta-sigma ADCs, however, the signal-to-noise ratio (SNR) performance and dynamic range can be greatly diminished. The dynamic range of a system may be described as the range of amplitudes between a noise component of the system and the onset of clipping (the level at which the power supply is no longer adequate to provide larger waveforms), while the SNR may be described as the ratio of a signal component (at some arbitrary level) to a noise component.
Many audio applications require enhanced SNR performance and dynamic range while maintaining particular design specifications, such as the operating voltage and the oversampling ratio. Other considerations involve the cost of manufacturing the ADCs, as an increase in the chip area increases the total cost of the ADC.
To improve the SNR of the delta-sigma ADC, the noise within the system must be reduced and/or the signal must be increased. Noise that is commonly associated with delta-sigma ADCs employing switched capacitor type integrators is kT/C noise, where k, T and C represent the Boltzmann constant, absolute temperature, and capacitance value, respectively. This noise describes the total thermal noise power added to a signal when a sample is taken on a capacitor. One way to reduce this noise is to increase the capacitance. Since the capacitance of a capacitor may be approximated with the equation C=∈A/d (where C is the capacitance in Farads, ∈ is the permittivity of the dielectric, A is the area of plate overlap in square meters, and d is the distance between the plates in meters), the capacitance may be increased by increasing the area A. Increasing the capacitance using this method, however, results in a larger time constant, causing slower switching operation.
Operational amplifiers (“op-amps”), which are commonly employed in delta-sigma ADCs, are also responsible for generating noise, such as flicker noise (also referred to as “1/f noise”) and thermal noise. Reducing the noise generated by the op-amp may further improve the SNR of the delta-sigma ADC. Meeting the required specifications of the system, however, such as a specified oversampling ratio and operating voltage, while achieving high performance in the op-amp is difficult, at least in part because all frequency bands may be necessary for the sampling system (such as in a switched capacitor configuration). Further, reducing the op-amp noise by conventional methods leads to transistors that occupy very large areas on the IC, which increases the current and power consumption considerably.